As shown in FIG. 1, an air-fuel ratio control apparatus has been widely known, wherein the apparatus includes a three-way catalyst (53) disposed in an exhaust passage of an internal combustion engine, an upstream air-fuel ratio sensor (67) and a downstream air-fuel ratio sensor (68) disposed upstream and downstream of the three-way catalyst (53), respectively.
This air-fuel ratio control apparatus calculates an air-fuel ratio feedback amount based on outputs of the upstream and downstream air-fuel ratio sensors such that the air-fuel ratio of the air-fuel mixture supplied to the engine (air-fuel ratio of the engine) coincides with the stoichiometric air-fuel ratio, and feedback-controls the air-fuel ratio of the engine based on the air-fuel ratio feedback amount. Furthermore, there has been also widely known an air-fuel ratio control apparatus which calculates an “air-fuel ratio feedback amount to have the air-fuel ratio of the engine coincide with the stoichiometric air-fuel ratio” based on the output of the upstream air-fuel ratio sensor only, and feedback-controls the air-fuel ratio of the engine based on the air-fuel ratio feedback amount. The air-fuel ratio feedback amount used in each of those air-fuel ratio control apparatuses is a control amount commonly used for all of the cylinders.
Incidentally, in general, an electronic-fuel-injection-type internal combustion engine has at least one fuel injection valve (39) at each of the cylinders or at each of intake ports communicating with the respective cylinders. Accordingly, when the characteristic/property of the fuel injection valve of a certain specific cylinder changes to a “characteristic that the injection valve injects fuel in an amount excessively larger than an instructed fuel injection amount”, only the air-fuel ratio of an air-fuel mixture supplied to that certain specific cylinder (the air-fuel ratio of the specific cylinder) greatly changes toward a rich side. That is, the degree of air-fuel ratio non-uniformity among the cylinders (inter-cylinder air-fuel ratio variation; inter-cylinder air-fuel ratio imbalance) increases. In other words, there arises an imbalance among “cylinder-by-cylinder air-fuel ratios”, each of which is the air-fuel ratio of the air-fuel mixture supplied to each of the cylinders.
In such a case, the average of the air-fuel ratios of the air-fuel mixtures supplied to the entire engine becomes an air-fuel ratio in the rich side in relation to (with respect to) the stoichiometric air-fuel ratio. Accordingly, by the air-fuel ratio feedback amount commonly used for all of the cylinders, the air-fuel ratio of the above-mentioned certain specific cylinder is changed toward a lean side so as to come close to the stoichiometric air-fuel ratio, and, at the same time, the air-fuel ratios of the remaining cylinders are changed toward a lean side so as to deviate from the stoichiometric air-fuel ratio. As a result, the average of the air-fuel ratios of the air-fuel mixtures supplied to the entire engine is made substantially equal to the stoichiometric air-fuel ratio.
However, since the air-fuel ratio of the specific cylinder is still in the rich side with respect to the stoichiometric air-fuel ratio and the air-fuel ratios of the remaining cylinders are in the lean side with respect to the stoichiometric air-fuel ratio, combustion of the air-fuel mixture in each of the cylinders fails to become complete combustion. As a result, an amount of emissions (an amount of unburned combustibles and an amount of nitrogen oxides) discharged from each of the cylinders increases. Therefore, even when the average of the air-fuel ratios of the air-fuel mixtures supplied to the the engine is equal to the stoichiometric air-fuel ratio, the increased emissions cannot be completely removed/purified by the three-way catalyst. Consequently, the amount of emissions may increase.
Accordingly, in order to prevent emissions from increasing, it is important to detect a state in which the degree of air-fuel ratio non-uniformity among the cylinders has been excessively large (occurrence of an inter-cylinder air-fuel ratio imbalance state) to take some measures against the imbalance state. It should be noted that, an inter-cylinder air-fuel ratio imbalance also occurs, for example, in a case where the characteristic of the fuel injection valve of the certain specific cylinder changes to a characteristic that it injects fuel in an amount excessively smaller than the instructed fuel injection amount.
One of conventional apparatuses for determining whether or not such an inter-cylinder air-fuel ratio imbalance state has occurred is configured so as to obtain a trace/trajectory length of an output value (output signal) of an air-fuel ratio sensor (the above-mentioned upstream air-fuel ratio sensor 67) disposed at an exhaust merging/aggregated portion/region into which exhaust gases from a plurality of the cylinders of the engine merge, compare the trace length with a “reference value which changes in accordance with the rotational speed of the engine,” and determine whether or not the inter-cylinder air-fuel ratio imbalance state has occurred on the basis of the result of the comparison (see, for example, U.S. Pat. No. 7,152,594).
It should be noted that, in the present specification, the expression “inter-cylinder air-fuel ratio imbalance state (excessive inter-cylinder air-fuel ratio imbalance state)” means a state in which the difference between the cylinder-by-cylinder air-fuel ratios (cylinder-by-cylinder air-fuel ratio difference) is equal to or greater than an allowable value; in other words, it means an inter-cylinder aft-fuel ratio imbalance state in which the amount of unburned combustibles and/or nitrogen oxides exceeds a prescribed value. The determination as to whether or not an “inter-cylinder air-fuel ratio imbalance state” has occurred will be simply referred to as “inter-cylinder air-fuel ratio imbalance determination” or “imbalance determination.” Moreover, a cylinder supplied with an air-fuel mixture whose air-fuel ratio deviates from the air-fuel ratio of air-fuel mixtures supplied to the remaining cylinders (for example, an air-fuel ratio approximately equal to the stoichiometric air-fuel ratio) will also be referred to as an “imbalanced cylinder.” The air-fuel ratio of the mixture supplied to the imbalanced cylinder will also be referred to as an “air-fuel ratio of the imbalanced cylinder.” The remaining cylinders (cylinders other than the imbalanced cylinder) will also be referred to as “normal cylinders” or “balanced cylinders.” The air-fuel ratio of the mixture supplied to the normal cylinder will also be referred as an “air-fuel ratio of the normal cylinder” or an “air-fuel ratio of the balanced cylinder,”
In addition, a value (e.g., the above-mentioned trace length of the output value of the air-fuel ratio sensor), whose absolute value becomes larger (monotonously) as the cylinder-by-cylinder air-fuel ratio difference (the difference between the air-fuel ratio of the imbalanced cylinder and those of the normal cylinders) becomes larger, and which is obtained based on the output value of the air-fuel ratio sensor in such a manner that the absolute value becomes larger as a fluctuation of an air-fuel ratio of an exhaust gas reaching the air-fuel ratio sensor becomes larger, will also be referred to as an “air-fuel ratio fluctuation indicating amount.” In addition, a value, which becomes larger as an absolute value of the air-fuel ratio fluctuation indicating amount becomes larger, and which is obtained based on the air-fuel ratio fluctuation indicating amount will also be referred to as an “imbalance determination parameter.” This imbalance determination parameter is compared with an imbalance determination threshold to carry out the Imbalance determination.